1. Field of the Invention
The present invention relates generally to bearings for alternators and bearings for pulleys, and particularly to such bearings less prone to flake-accompanied, hydrogen embrittlement or cracking in an ambient of hydrogen.
2. Description of the Background Art
In bearings used for alternators or pulleys there is often observed a unique fracture accompanied with flakes at its damage starting point. The flake is dissimilar to a white etching constituent (WEC) introduced by rolling fatigue, a so called butterfly introduced in the vicinity of a non-metal inclusion and the like. The present flake is characterized by failing to have directivity relative to a direction of rolling. A bearing having a damage accompanied with flakes at a starting point apparently shows an increased amount of hydrogen in the steel, and in the flake there is a crack, which extends along a grain boundary deep inside. As such, such a damage as described above has been considered to be attributed to hydrogen. Hereinafter, this flake-accompanied, unique fracture will be referred to as hydrogen embrittlement or cracking.
Hydrogen embrittlement or cracking is caused possibly because as the bearding rotates, a chemically active fresh metal surface is produced and its catalysis effect decomposes a lubricant and generates hydrogen, which enters into the steel. Accordingly, as an approach against hydrogen embrittlement or cracking, (a1) using a chemically less decomposable lubricant and (a2) blackening to minimize the exposure of fresh metal surface have been considered effective.
For item (a1), a bearing is proposed, as follows: alkyldiphenylether oil and poly α olefin oil are blended at a ratio in weight of 20:80 to 80:20 to prepare a base oil, which is blended with 5-40% by weight of an aromatic diurea compound or an aromatic urea-urethane compound as a thickener, and furthermore a grease composite with a passivation oxidizer and organic sulfonate added thereto is enclosed in a roller bearing to provide a grease enclosed bearing for an alternator, as disclosed in Japanese Patent Laying-open No. 5-263091.
For item (2b), a grease enclosed bearing is proposed that has a raceway having a rolling contact surface covered with an oxide film of 0.1 to 2.5 μm in thickness, as disclosed in Japanese Patent Laying-open No. 2-190615.
The above proposals, as confirmed in the field of the art, provide effectively prolonged lives.
In recent years, however, bearings for alternators and those for pulleys are used in increasingly severe environments, and simply taking the above approach is no longer sufficient. There is a demand for an approach that can provide an increased life against hydrogen embrittlement or cracking.